Earlier in 2018 the U.S. military confirmed a rare problem with its M4 and M16 assault rifles that results in the weapon firing without pulling the trigger. The flaw is encountered rarely because less than eight percent of all M4s and M16s have it and even then it only occurs under specific conditions. For those weapons with the mechanical defect, you must first have a round chambered and ready to fire. Then the safety switch must be between safe and fire. At that point, the weapon would not fire. If the user then moved the adjacent semi-automatic/full automatic switch to switch to either mode the rifle would fire without the trigger being pulled.

Further research found that for the six most common models of the M16/M4 rifles the flaw rate varied from zero (for a basic M16A2) to nine percent (for M4A1 PIP). Further investigation found that up to 900,000 weapons would have to be inspected and, if the presence of the flaw was confirmed, repaired (by replacing selector switches with tolerance defects). The inspection and repair process for army weapons will take up to 18 months to complete.

The M16 design has had several problems since it was introduced in the 1960s. Since the 60s over 20 million have been produced and many times more if you count foreign clones of the M16 design. In the past, the main complaint with the M16 design was the tendency to jam if not kept clean. That has been a very noticeable and much complained about flaw. This new misfire problem actually occurs very rarely and although it has existed for years was, until recently, was just considered another random misfire problem which, in combat, was not considered something unique. Currently, the M4 (short barrel version of the M16) is the most frequently used combat version of the M16. There are still a lot of users would like to see a new weapon but so far no one has been able to agree on a replacement sufficiently superior to justify the effort and expense.

The M16/M4 assault rifle has long been controversial mainly because of the original gas-tube system to power the automatic reloading and loading system. The gas-tube system required more frequent cleaning to prevent fouling of the barrel and failure of the reloading system. There were a lot of other minor problems but these have been gradually addressed.

The main problem is that the U.S. Army will not make major (and expensive) changes to the M4. In 2015 the army agreed to implement another batch of upgrades for its existing M4A1, basically creating the M4A2 (or M4A1+, take your pick). The list of M4A2 improvement was still being compiled from user suggestions and analysis of what is affordable when the effort was canceled in 2016 because of budget problems. Some of the most popular suggested improvements were a longer (by a third) Picatinny rail (to make some accessories easier to use), a floating barrel, a single stage trigger (preferred by snipers), a more effective flash suppressor, camouflage-colored components (to make the currently all black rifle less easy to spot), better cleaning tools (more like commercially available stuff) and some design changes (different shapes and materials for some components plus more efficient slings) to make the M4 easier to handle and carry. The "floating" means that the barrel is attached only to the main body of the rifle to reduce resonance (which throws off accuracy.) The new trigger would only be on M4s used by marksmen (about ten percent of all infantry) who are trained to be snipers.

The M4A2 effort came after yet another failure to find a replacement rifle. In 2013 the army held a competition to find a replacement for the aging (1980s design that entered wide service in 1994) M4 carbine. There has been a lot of pressure from the troops and Congress (where many of the complaints end up, after all, the M4 users are also voters). The basic infantry weapon, the M16, is over half a century old and the M4 is a shortened version of that. The rifle the M16 replaced (the M14) lasted less than a decade and the one before that (the M-1) lasted two decades. The one before that (M1903) lasted three decades and so on, back to the Civil War (1861-65). The M4 is a variant of the M16 that was widely adopted in the 1990s and there are still plenty of M16s in service. The army declared the competition inconclusive and went for yet another upgrade of the M4, which was later canceled.

Meanwhile, the army has been upgrading its older M4 assault rifles to the M4A1 standard since 2011. Ultimately upgrade kits were purchased for 70 percent of the army’s half-million M4 carbines and that turns them into M4A1s. At the current rate, this will take until 2020. The kits replace the barrel, receiver, and auto-loading system with one that is easier to keep clean. There is also a heavier barrel and the ability to fire full automatic. There is also a stronger Picatinny rail on top of the barrel, for mounting scopes and such.

Most elements (except for the short stroke piston system) of the M4A1 were already incorporated by SOCOM for their own M4s, which were, in effect, the first M4A1 models to enter service. The marines are not upgrading their M4s (which are mainly used by support troops).

This conversion kit addressed years of complaints about the M4 and M16 assault rifles. The main change was replacing the main portion of the rifle with a new component that contains a short stroke piston gas system (to reduce buildup of carbon inside the rifle) and a heavier (by 142 gr/five ounces) barrel (which reduces barrel failure from too much heat, which happens when several hundred rounds are fired within a few minutes).

The user demand for the adoption of the short stroke piston gas system has been around for decades so that M16s would be more reliable and easier to clean. This came to a head (again) in 2007 when the army ran more “dust and reliability” tests on its M4 rifle as well as several similar rifles. The four weapons tested were: the M4, the XM8, SCAR (Special Operations Forces Combat Assault Rifle), and the H&K 416 (similar to the new M4 with the upgrade kit and short stroke piston system).

The testing consisted of exposing the weapons to 25 hours of heavy dust conditions over two months. During that testing period, 6,000 rounds were fired from each of ten weapons of each type. The weapons with the fewest failures (usually jams) were rated highest. Thus, the XM8 finished first, SCAR second, 416 third, and M4 last. In response, the army said it was satisfied with the M4's performance but was considering equipping it with a heavier barrel (to lessen overheating) and more effective magazines (27 percent of the M4s 882 jams were magazine related). The army noted that the M4 fired over 98 percent of its rounds without problems. That missed the point that the other rifles had far fewer jams. In combat, each jam is a life-threatening situation for the soldier in question. The army had been forced by Congress to conduct the tests. Congress was responding to complaints by the troops. But the army refused to adjust its budget priorities to act on the test results.

The XM8 had 127 jams, the SCAR 226, and the 416 had 233. Thus the M-4 had nearly eight times as many jams as the XM8, the rifle designed to replace it. The M4 had nearly four times the jams of the SCAR and 416, which were basically M4 type rifles with a short stroke piston system. Any stoppage is potentially fatal for the soldier holding the rifle, thus the disagreement between the army brass and the troops who use the weapons in combat.

In dusty places like Iraq and Afghanistan you have to clean your M16 and M4 rifles constantly, otherwise the combination of carbon (from the recoil system) and dust in the chamber will cause jams. The army and marines both decided to stick with their current weapons, rather than adopt an easier to maintain the weapon, like the XM8 or H&K 416, because of the billion or so dollars it would cost to switch rifles.

If the issue were put to a vote the troops would vote for a rifle using a short-stroke system (like the XM8, SCAR, or H&K 416). But the military is not a democracy, so the troops spend a lot of time cleaning their weapons and hoping for the best. The debate involves two intertwined attitudes among senior army commanders. First, they don't want the hassle, and possible embarrassment, of switching to a new rifle that might have even more difficult problems. Second, they are anticipating a breakthrough in weapons technology that will make possible a much-improved infantry weapon. This is likely to happen later, rather than sooner, but the generals kept obsessing over it. They are encouraged by recent success in the development of caseless and short case ammo for a new machine-gun design.

Earlier efforts to just get the troops a more reliable rifle have failed. Back in 2005, the U.S. Army's design for a new assault rifle, the XM8, was canceled. After that, the manufacturer incorporated one of the key components of the XM8 into M4 rifles and calls the hybrid the H&K 416. Heckler & Koch (H&K) designed the XM8, which was based on an earlier H&K rifle, the G36. SOCOM is using the 416 but no one else is (except for a few police departments).

The XM8 (like the G36 and 416) uses a short-stroke piston system. The M16 uses the gas-tube system, which leads to carbon build up and jams (rounds getting stuck in the chamber and the weapon unable to fire). The short-stroke system also does not expose parts of the rifle to extremely hot gases (which wears out components more quickly). As a result, rifles using the short-stroke system, rather than the gas-tube, are more reliable, easier to maintain, and last longer.

H&K developed the 416, for SOCOM, at the same time the XM8 was being evaluated by the army. SOCOM got the first 416s in 2004, a year before the Army canceled the XM8. The 416 looks like the M4, for the only thing that has changed is the use of the short-stroke system to automatically extract the cartridge after the bullet has been fired and loads the next round. SOCOM can buy pretty much whatever they want, the U.S. Army cannot. SOCOM listens to what its troops want, the army often doesn't.

The army proposed some other changes, as part of the M4 component replacement. These included improved trigger pull characteristics, ambidextrous controls (to make life easier for lefties), and a round counter (in the pistol grip) to track the number of bullets fired over the lifetime of the rifle (makes for better data on how rifles perform over time and for scheduling the replacement of components). Few of these were implemented because of tight budgets after 2010.

The main problem with replacing the M4 is that the weapon is not dramatically inferior to most proposed replacements. Moreover, troops acknowledge it is the “devil they know” and will all the minor changes and new accessories that have become available since 2001 the M4 has managed to keep up. Army leaders point out that with constantly shrinking defense budgets you have to be careful with what you spend your money on. Replacing the M4 and M16 would cost over $5 billion and you might, as has happened to many other countries, get a weapon that looks good on paper (and in pictures) but reveals some serious flaws once it is exposed to sustained combat. That’s what makes most combat troops (especially infantry) willing to stick with an improved M4.